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A gneiss complex in the Grouse Creek Mountains, northwestern Utah, consists of 2.5-b.y.-old adamellite that was remobilized and intruded younger Precambrian and Paleozoic cover rocks 25 m.y. ago during an extended period of synkinematic metamorphism. One or more structural domes formed in the region during metamorphism. The remains of a middle Tertiary culmination in the central Grouse Creek Mountains is an asymmetrical welt of schistose Precambrian adamellite that protrudes into greatly attenuated autochthonous and allochthonous cover rocks. Fine- to medium-grained gneiss and schist of the upper 200 m of the welt, or dome, grade upward to retrograde phengite-quartz-albite schist that intruded the lowermost beds of upper Precambrian(?) quartzite. The age of the schistose Precambrian adamellite and its geographic continuity with less-metamorphosed Precambrian adamellite lying unconformably beneath the quartzite indicate that remobilization of the former was post-Precambrian. Simultaneously, cover rocks were thinned to one-fifth their original thickness by metamorphic flattening, extension, and low-angle faulting.

A 25-m.y.-old pluton underlies the area of remobilization. Its outer shell bears metamorphic folds related to a second deformation. These folds also predominate in a metasomatic aureole in the Precambrian adamellite surrounding the Tertiary pluton and in the upper, schistose part of the gneiss dome. Thus, development of the dome was closely related to the second deformation and to the intrusion 25 m.y. ago.

Late-metamorphic folds and low-angle faults chiefly affected allochthonous cover rocks, and postmetamorphic movements carried parts of these rocks over 1 l-m.y.-old sedimentary rocks in the adjoining basin. Isotopic data on autochthonous rocks suggest that metamorphic temperatures persisted into Miocene time and provided the potential for continuing uplift and basinward shedding of sheets which ended after 11 m.y. ago.

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